New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/35 Miocene stratigraphic relations and problems between the Abiquiu, Los Pinos, and Tesuque Formations near Ojo Caliente, northern Espanola Basin S. Judson May, 1984, pp. 129-135 in: Rio Grande Rift (Northern New Mexico), Baldridge, W. S.; Dickerson, P. W.; Riecker, R. E.; Zidek, J.; [eds.], New Mexico Geological Society 35th Annual Fall Field Conference Guidebook, 379 p. This is one of many related papers that were included in the 1984 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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JUDSON MAY Arco Exploration Company, Box 5540, Denver, Colorado 80217 INTRODUCTION near Ojo Caliente to over 300 m to the west and southwest in the The complex stratigraphic relations of the Abiquiu Formation, the Abiquiu area. The basal contact with the pre-Miocene rocks may drop mid-Miocene part of the Los Pinos Formation, and the Tesuque For- as much as 200 m because of pre-Abiquiu topography, allowing for the mation are locally well exposed in the northern Espanola Basin near greater thickness to the west. Ojo Caliente (Fig. 1). Exposures in this area show that much of the Los Pinos in the Ojo Caliente—Petaca area is equivalent in stratigraphic Contacts level and age to the Chama—El Rito Member of the Tesuque Formation Throughout most of the western Espanola Basin the Abiquiu uncon- (Fig. 2) and most or all of the Abiquiu Formation in the Espanola Basin. formably overlies pre-Miocene rocks. In the hills west of Ojo Caliente In the Ojo Caliente area the Abiquiu and Los Pinos represent a large the Abiquiu overlies a mature, early Miocene topography eroded on southwest-sloping fan complex of volcaniclastic conglomerate and Precambrian crystalline rock (see third-day road log, this guidebook). sandstone derived from rhyolitic and andesitic terranes to the north and This surface locally exhibits up to 300 m of relief. The Abiquiu probably northeast (Manley, 1981). The Tesuque Formation records a large ar- represents the initial major influx of elastic debris that filled the Espanola kosic accumulation of sand carried mainly west and south from Pre- Basin. cambrian rocks of the Sangre de Cristo uplift. In the Ojo Caliente area Smith (1938) and Smith and others (1961) recognized a gradational these two alluvial systems merged and intertongued in a complex man- upper contact of the Abiquiu with the overlying Santa Fe Formation. ner, producing the Chama—El Rito Member of the Tesuque Formation However, in the Ojo Caliente area, the contact they recognized is which contains interbedded volcaniclastic gravels and arkosic sand- considered here to be the upper contact of the Los Pinos Formation. stones of the two systems. Under the more restricted definition of the Abiquiu used here, the Abiquiu grades upward into the Los Pinos in the Ojo Caliente area. To ABIQUIU FORMATION the south and southwest it probably grades upward into the Chama—El The Abiquiu Formation (Abiquiu Tuff of Smith, 1938) consists of Rito Member of the Tesuque Formation of the Santa Fe Group (Fig. light-gray, tuffaceous sandstone and minor volcaniclastic conglomerate 2), although this relationship can be seen only west of El Rito Creek. exposed near the town of Abiquiu and distributed widely throughout Galusha and Blick (1971) also reported a slight angular unconformity the Abiquiu 30-min quadrangle. Only the uppermost part of the Abiquiu between the Abiquiu and Chama—El Rito near the "Ojo Caliente intru- is exposed at Ojo Caliente and is composed of andesite- and rhyolite- sive" southwest of the town, but the beds under the unconformity at pebble conglomerate containing sparse pebbles of Precambrian quartz- this location are the tuff-ring facies of the much younger Ojo Caliente ite, metarhyolite, gneiss, and schist. The Abiquiu of this report is tuff ring (May, 1980) and are located in the middle part of the ChamaEl restricted to the lower 0-80 m of the roughly 420 m mapped as Abiquiu Rito (Figs. 1, 2). by Smith (1938) in the Ojo Caliente area. The remaining upper 360 m In the Ojo Caliente area the only exposure of the upper contact is 3 of volcaniclastic conglomerate and tuffaceous sandstone is considered km west of town. Here the formation grades up into the Los Pinos to be the Los Pinos Formation (Cordito Member?) based on its strati- through a 40-m section. The very light-gray tuffaceous sandstones grad- graphic position and approximate lithologic similarity with the Cordito ually turn darker gray and the rhyolite- and andesite-pebble conglom- to the north. erates gradually increase to the point where they make up a major part of the section. The contact is chosen at approximately 15 m above the Lithology lowest volcanic-pebble conglomerate or where the conglomerates make Near Ojo Caliente, the Abiquiu contains mainly white, very light- up more than 25% of the section. Although a few of the conglomerates gray, light-brown, and minor red and green layers of tuffaceous sand- are locally erosional at their bases, the contact exhibits neither signif- stone. In the upper 20 m are a few layers of andesite- and rhyolite- icant erosion nor angularity between the underlying and overlying beds. pebble conglomerate, sparse fluvial crossbeds, laminar beds and chan- nel-fill structures. Sparse, discontinuous seams of mudstone occur lo- Age cally. In the lower 40 m, several dark lenses of conglomerate and breccia The Abiquiu ranges from 22 to 17-18 m.y. in age (early Miocene) contain subangular cobbles and fragments of locally derived Precam- near Ojo Caliente. Basalt flows of the Hinsdale Formation lie between brian metarhyolite. These lenses are lithologically similar to the Ritito the Abiquiu and the underlying Precambrian rocks along the north wall Conglomerate north of La Madera. The best local exposure is 3 km of Arroyo El Rito and have been K—Ar dated by Baldridge and others west—southwest of Ojo Caliente in a 70 m section along the north wall (1980) at 22.1 ± 0.6 m.y. These flows intertongue with thin discon- of Arroyo El Rito (Fig. 1). tinuous seams of light-gray tuffaceous sandstone, indicating Abiquiu The sandstone is variable in composition, consisting of medium to deposition during eruption of the flows. However, the lower part of the coarse-grained, subangular to subrounded quartz, K-feldspar and pla- Abiquiu in its type area is probably lower stratigraphically and older gioclase, with a few granular fragments of Tertiary sedimentary and than the Abiquiu in the Ojo Caliente hills, implying that it is also partly volcanic rocks and Precambrian metamorphic rock. Traces of opaque upper Oligocene. Vazzana (1980) inferred a 27-26 m.y. age for the minerals and altered biotite flakes also occur. Larger angular fragments base of the formation to the west. and granules of metarhyolite and pink feldspar occur in minor amounts Age of the upper contact is well constrained locally. Four km west throughout the sandstone, but generally decrease upward in size and of Ojo Caliente, a small olivine-nephelinite flow on the south slope of number. The fine-grained interstitial debris in most layers has a tuffa- Cerro Negro yielded a K—Ar date of 18.9 0.7 m.y. (Baldridge and ceous appearance. The Abiquiu appears to thicken greatly from a maximum of 90 m others, 1980); this flow occurs approximately 40 m below the top of roon and bluish-gray rhyolite and rhyolitic welded tuff, containing eu- the Abiquiu, implying an approximate 18 m.y. age for the top. Manley taxitic structure and phenocrysts of quartz. Clasts of light-gray quartz and Mehnert (1981) reported a 17.3 ± 0.8 m.y. K—Ar age on a dacitic latite(?) and andesite porphyry are also abundant; both contain phe- clast approximately 10 m below the top of the Abiquiu Formation 7 nocrysts of hornblende, light-gray feldspar, and sparse biotite and quartz.